1. Field of the Invention
The present invention relates to an optical disc as an information recording and reproducing medium for optically recording and reproducing information, in which a light permeable layer is mounted on a substrate, and a laser beam is irradiated from a side of the light permeable layer, not passing through the substrate.
The present application claims priority from Japanese Application No. 2000-169133, the disclosure of which is incorporated herein by reference for all purposes.
2. Description of the Related Art
A conventional optical disc has such a structure that is provided with a replica substrate with a concave and convex surface on a single side thereof , made of plastic with the light permeability, a signal recording surface having a reflective membrane and a recording membrane on the concave and convex surface, and a protective membrane formed on the signal recording surface, so that the signal recording surface can be sufficiently protected, and also a manufacturing step of the disc can be simplified. In this conventional optical disc, a laser beam is irradiated from a side of a substrate surface, and passes through the substrate to record and reproduce information. Since the substrate of such an optical disc is a layer through which the laser beam passes, it is required to thin the thickness of the substrate in order to improve a recording density thereof. However, the above mentioned substrate is usually formed by an injection molding, so that there is a limitation in thinning the substrate.
On the other hand, recently, an optical disc having an light permeable layer made from a thin membrane on a signal recording surface of a substrate is proposed, in which a laser beam is irradiated from a side of the light permeable layer, and does not pass through the substrate to record and reproduce information. In this case, the thickness of the substrate has no relation with realizing a high recording density of the disc since the substrate does not operate as the light permeable layer. Therefore, the substrate may have a normal thickness, in which the concave and convex surface can be exactly transcribed on an upper surface of the substrate by the injection molding, since the substrate need not be thinned.
In the recently proposed optical disc, whereas the thickness of the substrate can be freely designed, it becomes a serious problem how the light permeable layer is formed since durability of the light permeable layer with regard to the thickness and forming condition of the layer has a large effect upon recording and reproducing characteristics.
In addition, the conventional optical disc has such a problem that a peripheral end of the substrate is apt to be damaged since the disc is usually held by gripping together the peripheral end of a central hole of the substrate and an outer peripheral end of the substrate. In the above recent optical disc with the light permeable layer, such a problem has not yet been discussed. Thus, there is a fear that a deterioration such as tearing off of the light permeable layer may be caused in the peripheral end when forming a light permeable membrane extending to the peripheral end.
The present invention aims to overcome these conventional problems, and an object of the present invention is to provide an optical disc having a light permeable layer formed on a signal recording surface, in which an light permeable layer is formed on a signal recording surface of a substrate, and a laser beam is irradiated from a side of the light permeable layer to record and reproduce information. The optical disc is designed so as to form the uniformly thin light permeable layer with an improved durability, resulting in a high quality thereof.
In order to achieve the above objects, there is provided an optical disc according to the present invention, which comprises a substrate having a disk-like shape, a signal recording surface formed on the substrate, and a light permeable layer stacked up on the signal recording surface with a circular shape. In the optical disc, a laser beam of wave-length 390-440 nm is irradiated onto the signal recording surface through an objective lens of a numerical aperture 0.8-0.9 and then the light permeable layer to record and/or reproduce information, and a diameter size of the light permeable layer is set to be smaller than one of the substrate.
According to this feature with regard to a size of the light permeable layer, the transparent sheet has no effect of an outer force even if the outer force, for example, by fingers is applied to the outer peripheral end of the substrate, so that the transparent sheet can maintain an original good condition thereof.
Furthermore, the present invention has the following features in addition to the above-mentioned features. Namely, the light permeable layer comprises a transparent sheet, and further a protective layer is formed so as to cover an surface of the substrate exposed between an outer peripheral end of the transparent sheet and an outer peripheral end of the substrate. Also, a membrane thickness of the transparent sheet is formed to be 10-350 xcexcm, and a thickness of the substrate is formed to be more than 0.3 mm. Moreover, the substrate may be colored for discriminating a kind of discs.
According to these features, the protective layer can further improve durability of the transparent sheet. Furthermore, the above-mentioned thickness, i.e., 10-350 xcexcm, of the transparent sheet allows the transparent sheet to effectively function as both a protective layer and a light permeable layer so that a high density of optical disc can be formed, and also the thickness, more than 0.3 mm, of the substrate allows the substrate to be strengthened.
In addition, the coloring of discs allows the optical disc to be discriminated according to a kind of disc, for example, a reproducing only disc, a writable disc, and a re-writable disc through applying a different color to each kind of disc, so that a convenience of usage can be improved.